Supported chromium oxide catalysts have become an important vehicle for polymerization of 1-olefins such as ethylene and predominantly ethylene copolymers. As originally commercialized, these polymerizations were carried out under solution conditions. However, it was early recognized that a more economical route to such polymers was to utilize a slurry system, sometimes called a particle form system, wherein the polymerization is carried out at a low enough temperature that the resulting polymer is insoluble in the diluent. It is highly desirable in slurry polymerization systems to achieve relatively high productivity. This goal is hampered to some extent by the fact that silica supported chromium oxide catalysts tend to have an induction period under slurry conditions. This is particularly disadvantageous in a slurry system utilizing a loop type reactor with a settling leg for polymer recovery.